This invention relates in general to an electrophotographic imaging system, and more specifically, to an electrophotographic imaging member containing a hole injection layer and a method of utilizing such device.
The formation and development of images on the imaging surfaces of electrophotographic imaging members by electrostatic means is well known. One of the most widely used processes being xerography described, for example, in U.S. Pat. No. 2,297,691. Numerous different types of photoreceptors can be used in the electrophotographic imaging process. Such electrophotographic imaging members may include inorganic materials, organic materials, and mixtures thereof. Electrophotographic imaging members may comprise contiguous layers in which one of the layers performs a charge generation function and the other layer forms a charge carrier transport function or may comprise a single layer which performs both the generation and transport functions. These electrophotographic imaging members may be coated with a protective overcoating to improve wear. For Carlson type electrophotographic imaging processes, the protective overcoating must allow the electrostatic charge initially deposited on the outer surface of the overcoating to form at the interface between the protective overcoating and the underlying photoconductive layer prior to repeating the next imaging cycle. Protective overcoatings may be of various organic and inorganic materials including resins, photoconductive materials and the like.
Electrophotographic imaging members based on amorphous selenium have been modified to improve panchromatic response, increase speed and to improve color copyability. These devices are typically based on alloys of selenium with tellurium. The selenium electrophotographic imaging members may be fabricated as single layer devices comprising a selenium-tellurium alloy layer which performs both charge generation and charge transport functions. The selenium electrophotographic imaging members may also contain multiple layers such as, for example, a selenium alloy transport layer and a contiguous selenium-tellurium alloy generator layer. These selenium-tellurium alloys are characterized by a tendency to thermally generate free holes in the dark. The thermal generation of free holes in the dark is a source of undesirable dark decay. In electrophotographic imaging systems employing AC or negative corotron devices for the erase step of the conventional electrophotographic imaging cycle, the electrophotographic imaging member is not uniformly discharged to residual voltage during the xerographic cycle. When an electrophotographic imaging member which thermally generates free holes in the dark is used in such systems, sensitivity of the electrophotographic imaging device is enhanced during cycling. This enhancement is observed electrically during cycling as a decrease in background voltage in the regions corresponding to the exposed portions in the preceeding cycle and is visible as a loss of low density image reproduction capability and corresponding image voltage. In electrophotographic imaging systems where the image exposure step is such that the background voltage level approaches the residual voltage level, the electrophotographic imaging member sensitivity enhancement on the next cycle occurs in an imagewise fashion. Thus, the areas of the electrophotographic imaging member discharged to background voltage level show less device sensitivity enhancement than those portions at the dark development potential or at intermediate image voltage levels. The imagewise enhancement of device sensitivity is visible as a "ghost" on cycle n+1 of the image in relation to cycle n. In order to eliminate this imagewise sensitivity enhancement, neutralization of the bulk negative space charge in the photoconductive layer resulting from the thermal generation of free holes is required. However, in electrophotographic imaging systems utilizing AC corotron or negative corotron erase stations, this bulk neutralization does not occur. The ghosting appears as a negative image on subsequent copies and, of course, is unacceptable in automatic copiers, duplicators, and printers.